[Math] necessary condition for subspace of a vector space

linear algebravector-spaces

Currently I'm reading linear algebra books by Leon and Friedberg.
In Friedberg's book, to be a subspace, a subset of a vector space should
(1). contain zero vector,
(2). be closed under scalar multiplication and
(3). be closed under vector addition.

But condition (1) is missing in Leon's book.

I think (1) is not necessary since if (2) and (3) holds, then (1) must be true.

Is (1) necessary?

Thanks in advance.

Best Answer

Probably it depends on your definition of vector space (i.e.: do you consider $\emptyset$ to be a vector space?) In my opinion, $\emptyset$ is should not be considered a vector space for various reasons, e.g. the fact that $\operatorname{span}\emptyset=\{0\}$, and thus point (1) should be included in the axioms for vector subspaces.